Here is a good link concerning turbo/water jacket/coking.
Coking, turbo bearing lubrication and engine oil viscosity
The water jacket on a turbo is there to help with oil coking, it does not matter if your rig is gas or diesel the turbo is subject to heat coking, it is not just a left over gasser problem. The water jacket was designed by engineers to solve a problem that causes damage to your turbo.
The 13BT engine designed by Toyota has a turbo water jacket, the supply of water is above the thermostat in the outlet housing, both the supply and return are in the same housing the flow of water is based on the location and angle of the ports.
I find it amazing that people believe engineers put useless water jackets on a engine part and tell other people to block or ignore the design. Yes turbo's are made without water jackets and yes they work, yes turbo's are made with water jackets and they work, understanding the reason is the fun part.
Toyota located their water ports so the turbo doesn't over cool the oil in the turbo during start up allowing faster warm up, the water is supplied from above the thermostat, they supply a small amount to keep the oil from coking, this offers a measure of protection, Toyota also offers a turbo timer to help with the same problem for non gasser engines. If you monitor your heat buildup in the turbo and cool it down when needed then the water jacket is just good insurance, if you don't cool it down when needed than the water jacket will protect your turbo from a very real problem. Part of the problem is everyone has an opinion (so do I) but oil coking could care less what I think, different oils coke at different temps so stating a good temp for me (400deg F) pre turbo EGT (this one gets alot of debate) is my opinion 600 deg F or 200 deg C is 3 different opinions. Pick one and you bet your turbo and oil will agree with you if it does

if it doesn't

the engineers who designed the water jackets tried to help with

.
Just my .02 cents
Jim